As
the originator of the replacement wall tie, REDIFIX has focused on the remedial
market and is committed to a strategy of product development to satisfy
the needs of this sector. The company has a well-founded reputation for
high quality products and outstanding service. REDIFIX products are manufactured
within BS EN 1S09000.

Cavity
wall ties have been used in almost all types of construction throughout
the UK since the early 1900s - commercial, industrial and high-rise buildings,
as well as domestic housing.Their purpose was to join the inner and outer
leaves of the wall at pre-arranged centres, and be capa ble of transferring
loads between the two leaves. Often they were protected by a bitumen coating
or galvanising before installation.
Today, however, cavity wall tie corrosion is deemed to be a significant
structural defect. Initially, such tie failure was not recognised and, quite
often, the symptoms were treated without full investigation of the real
cause. Lateral cracking of the brickwork joint for example, was often attributed
to frost, storm damage or ground movement.
A wide variety of ties and mortars are susceptible, combined with age, climate
and environ mental exposure.As the mortar ages, alkalinity is reduced by
the absorption of atmospheric car bon dioxide.The presence of chlorides
or sulphates in the mortar mix hastens the passive layer breakdown and removes
any protective coating to the metal.The time period for this to occur varies,
but after as little as I 0 years, serious corrosion can occur.This corrosion
results in extreme expansion of the steel tie to four or five times its
original thickness.
Once corrosion is established, deterioration is comparatively rapid. Rust
can attain a crushing strength equal to concrete and it is well within the
power of corroded ties at normal spacing, to lift 5 metres of brickwork.
Such upward movement can result in an upturning of the roof, particu larly
at the gable end, to alleviate the pressure commonly known as the Pagoda
effect, or in bulging of the outer wall as the weight of the roof resists
upthrust. Other visible evidence of the problem, can be cracking around
windows or doors and at internal wall-to-ceiling junctions as floor joists
are pushed up by expansion.

It
is the condition of the outer wall that determines the remedial action to
be taken. In some cases, the wall must be rebuilt, tying it back to the
inner leaf. However, if the outer wall appears to be in good condition it
can be stabilised by retying. It was back in the 1970s that the first instances
of wall tie failure were identified. In South Wales, the use of black ash
mortar had made the problems particularly severe. REDHEAD/REDIFIX was called
in, and with the co-operation of the local authority in Newport, developed
the first REDHEAD/REDIFIX remedial wall tie.
In developing a system, a number of important criteria had to be met. Firstly,
ties had to be completely corrosion proof, which was solved by manufacturing
from 300 series 18/8 austenitic stainless steel.Also, ties should be capable
of being fixed externally to avoid re-decoration and disruption to tenants,
An effective moisture barrier should be incorporated, which was solved by
positioning a neoprene drip ring on the rod.
Taking into account these criteria, REDHEAD/REDIFIX have developed a wide
range of remedial products to cater for every application and all substrate
materials encountered in the construction of walls.

A
site survey must be made to ascertain the cause of any wall defects and
to establish the condition of existing walls. Wall failures can be attributed
to insuffient numbers of ties or badly installed ties, as well aswall tie
corrosion.The survey should analyse whether old ties should be removed or
isolated. The type of construction of both leavesof the wall carefully identified
to establish the best replacement wall tie for the remedial work required.

Variations
in substrate call for corresponding variations in remedial fixing techniques.Traditional
brick inner and outer leaves are usually re-tied by mechanical expansion
in each leaf.
The Mactie replacement tie comprises two parallel expander sleeves assembled
on a central rod, each being set to a pre-determined torque with a pre-set
torque device giving fast, controlled installation at low in-place cost.
Where the inner leaf is lightweight block, or suspect material incapable
of sustaining expansion stresses, secure fixing is achieved by injecting
BRC3 or styrene free resin. It is preferred that mechanical fixing is retained
in the outer leaf to allow vertical and horizontal differential movement
of the walls due to thermal movement.
The REDIFIX Product Selector recommends the most suitable product for the
site substrates concerned.

The location and density of replacement wall ties is the responsibility
of the structural engineer. Site tests should be carried out to obtain the
actualpullout values of the selected tie in the particular site substrate.
As ageneral guide, ties are fixed at 900mm horizontal centresand 450mm vertical
staggered to give a domino five pattern. Around openings the density is
normally increased to 300mm centres and max 225mm from opening.